This invention relates generally to operation of diesel engines in traction vehicles such as locomotives and relates more particularly to fuel control of diesel engines to reduce exhaust emissions.
Large self-propelled traction vehicles such as locomotives commonly use a diesel engine to drive an electrical transmission comprising generating means for supplying electric power to a plurality of electric traction motors whose rotors are drivingly coupled through speed-reducing gearing to respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine (typically a 16-cylinder turbo-charged diesel engine). When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of DC traction motors or inverted to controlled frequency AC power and applied to AC traction motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes full engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to full output, but the load on the engine must not exceed whatever level of power the engine can develop. Overloads can cause premature wear, engine stalling or "bogging," or other undesirable effects such as excess exhaust emissions. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and maximum, so that the engine develops whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control console located in the operator's cab of the locomotive. In addition to the eight conventional power notches, the handle has an "idle" position.
The position of the throttle handle determines the engine speed setting of the associated governor. In a typical electronic fuel injection governor system, the output excitation from a controller drives individual fuel injection pumps for each cylinder allowing the controller to individually control start of and duration of fuel injection for each cylinder. The governor compares the desired speed (as commanded by the throttle) with the actual speed of the engine, and it outputs signals to the controller to set fuel injection timing to minimize any deviation therebetween.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,000) are realized. Under normal conditions, the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipments.
The output power (KVA) of the main alternator is proportional to the product of the rms magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the magnitude of excitation current in the alternator field windings. For the purpose of accurately controlling and regulating the amount of power supplied to the electric load circuit, it is common practice to adjust the field strength of the traction alternator to compensate for load changes (traction motor loading and/or auxiliary loading) and minimize the error between actual and desired KVA. The desired power depends on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
A traction vehicle, such as a locomotive, also generally includes electrically resistive grid elements selectively connectable to the DC link. During operation of the locomotive, these grid elements are used to dissipate regenerative electrical energy from the locomotive motors so that the locomotive can be operated in a dynamic braking mode in which the motors, acting as generators, electrically retard motion of the locomotive. When the locomotive is at rest, the grid elements can be used for self-loading the locomotive power system for test purposes or to load the diesel engine in order to maintain a desirable operating temperature. En either braking or self-loading, the grid elements are controlled by the vehicle system controller switching a controlled value of resistance onto the DC link.
When the engine is operating, it is desirable to control fuel flow to the engine so that the horsepower developed matches the horsepower required by the electric generator and other equipment coupled to the engine. If the engine is over fueled, incomplete combustion may occur resulting in excessive exhaust emission of unburned hydrocarbons. If insufficient fuel is supplied, the engine will bog and may stall. Thus, it is desirable to provide fuel to the engine at a rate which will enable the engine to meet the power output requirements without having excess fuel that results in excessive exhaust emissions.
One method of monitoring exhaust emissions is to place a probe in the engine exhaust. The probe is connected to a conventional smoke meter which indicates the level of emissions in the exhaust. However, such a method increases vehicle cost and maintenance expense by adding more equipment which will require servicing. Accordingly, it is desirable to provide a method for controlling engine exhaust emission by using other normally monitored engine characteristics.